Scientists have discovered the
gene enabling multiple female morphs that give the Common Mormon butterfly its
very tongue-in-cheek name. Doublesex, the gene that controls gender in
insects, is also a mimicry supergene that determines diverse wing patterns in
this butterfly, according to a recent study published in Nature. The
study also shows that the supergene is not a cluster of closely linked genes as
postulated for nearly half a century, but a single gene controlling all the
variations exhibited by the butterfly's wings.

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Thanks to a medley of shapes,
sizes, colors and patterns, butterfly wing variations have helped scientists
understand evolution in numerous ways. Intense predator pressures have forced
butterflies to evolve different tactics to ward them off. Some butterflies have
evolved to feed on toxic plants, and consequently, are inedible and toxic
themselves. Predators think twice before attacking them. In response to this,
palatable butterflies have developed a rather devious tactic: they imitate the
wing patterns of toxic butterflies found in the region, a phenomenon known as
Batesian mimicry. Usually, females of such species mimic a toxic model, as they
are more prone to predation than males. Sexual dimorphism (marked differences
in gender) in mimetic butterflies is often a result of this, such as the Danaid
eggfly female, which imitates the Plain tiger.

In some cases, females of a
single mimetic species imitate not one, but several toxic species. This results
in multiple female forms or morphs: what scientists call polymorphism. The
widespread Asian butterfly, the Common Mormon (Papilio polytes) is a
perfect example of this. The male has a single non-mimetic wing pattern but the
female has four forms: a non-mimetic morph that resembles the male, and three
other forms that mimic the toxic butterflies in the genus Pachliopta found
around them. These females mimic the colors, wing patterns and even shape of
their Pachliopta models, right down to their small tail-like projections on the
hindwings.

This undoubtedly impressive
natural phenomenon drew the attention of geneticists in the 1960s. They
speculated that alternative wing patterns are controlled by a supergene, which
had to be a cluster of tightly linked genes located near each other. These
genes would control specific attributes of wing patterning such as the presence
or absence of tails, color differences on the forewing and so on.

So which supergene controls the
numerous wing pattern variations of the Common Mormon? What genes constitute
this supergene? And how do these genes produce the mimetic morphs? These were
the questions scientists sought to answer.